1. Field of the Invention
The invention relates to a brake cylinder device with a built-in automatic shoe clearance adjustment mechanism.
2. Summary of the Related Art
FIG. 12 shows a conventional brake cylinder device with a built-in automatic shoe clearance adjustment mechanism. This conventional hydraulically actuating cylinder, positioned between adjacent facing ends of brake shoes, functions to automatically adjust clearances between a brake drum and the brake shoes in addition to its function to separate the brake shoes apart and to restrict the returning positions of the brake shoes.
This brake cylinder device is designed to be bilaterally symmetrical except for some parts such as a locator 380 in the central portion; therefore, an arrangement of the device at the right half is mainly explained here. A cylinder body 100 has a large diameter bore 110 with a bottom, a partition 130 having a small diameter bore 120 which is coaxial to the large diameter bore 110, formed to be a fluid flow passage between opposed large diameter bores 110, 110 of the brake cylinder device via the small diameter bore 120. An adjustment gear 210 is formed at the periphery on the right end of a piston 200 stroking out from the large diameter bore 110, on which a piston head 220 is concentrically fit with a capacity to make a relative rotation. A notched groove 221 is formed at the right part of the piston head 220 to receive a shoe web of the brake shoe, not shown in FIG. 12.
An adjustment bolt 300 is engaged in non-reversible screw threaded connection with an internal thread of a coaxial blind-end hole of the piston 200. Here, xe2x80x9cnon-reversible screw threaded connectionxe2x80x9d means a screw threaded connection that does not cause relative rotation between the two members if a thrust force in the axial direction is transmitted on either one of the piston 200 or the adjustment bolt 300. A first clutch face 310 in a conical shape formed at the left end of the adjustment bolt 300 makes a clutch engagement with a corresponding clutch face formed at the halfway of the small diameter bore 120 in the partition 130.
A drive ring 320, the outer peripheral surface of which is beveled to provide a clutch face into engagement with a corresponding internal clutch face formed on a projection of the partition 130 at the entrance of the small diameter bore 120. Clutch engagements among the adjustment bolt 300, the drive ring 320, and the corresponding clutch faces of the cylinder body 100 are to be in conical shape in order to obtain a more stable rotational resistance than that of the clutch engagements with flat surfaces. The internal circumference of the drive ring 320 is provided with a fast thread, which is in mesh with a corresponding external thread 330 at the left side of the adjustment bolt 300 in a manner of reversible screw threaded connection with a slight gap (backlash hereinafter). Here, xe2x80x9creversible screw threaded connectionxe2x80x9d means a screw threaded connection that does cause relative rotation between the two members when a thrust force in the axial direction is applied on either one of the piston 200 or the adjustment bolt 300. An adjustment spring 340 provided between the adjustment bolt 300 and the drive ring 320 constantly urges the drive ring 320 in the direction to be into clutch engagement with the corresponding internal clutch face of the cylinder body 100 by its spring force.
A through hole 350 with a shaped large diameter bore 360 is formed inside the adjustment bolt 300 extending in its axial direction. The large diameter bore 360 is formed at the left part of the through hole 350 via a stepped surface 351, and there is a locator spring 370 positioned between facing two large diameter bores 360, 360 of the adjustment bolts 300, 300. This locator spring 370 is to prevent free movement due to vibration caused while in braking operation by acting an urging force to the adjustment bolts 300, 300. Moreover, the locator 380 and a spacer 390 are positioned between the right end of the locator spring 370 and the stepped surface 351, so that a torsion force of the locator spring 370 does not affect on both adjustment bolts 300, 300. Further, only the spacer 390 is positioned between the left end of the locator spring 370 and the stepped surface 351. The reference number 400 is a piston cup defining a hydraulic chamber 140, 410 is a backup ring, 420 is a dust boot sealing the large diameter bore 110, and 430 is an O-ring supporting the end of the adjustment bolt 300 in the side of the clutch face.
While in braking operation, upon pressurizing the hydraulic chamber 140 located at the bottom of the small diameter bore 120; the piston 200 moves the brake shoe outwardly into lining contact with the brake drum ultimately causing a braking effect. (It is noted that the shoe, lining, and drum are not shown in FIG. 12. These components are known to those of ordinary skill in the art and no further explanation is warranted.)
The operation of the automatic shoe clearance adjustment mechanism is explained hereunder. While in braking operation, the adjustment bolt 300 moves together with the piston 200 outwardly. Now, if the lining wears out and an amount of outward movement of the adjustment bolt 300 takes up and exceeds the backlash between the drive ring 320 and the adjustment bolt 300, the drive ring 320 is urged out of engagement with the corresponding clutch face and smoothly rotates.
When the brake is released and the adjacent brake shoe is retracted by the shoe return spring, (not shown in FIG. 12), the piston 200 and the adjustment bolt 300 return to the amount of the backlush, the drive ring 320 is urged once again strongly into clutching engagement disabling the rotation thereof, and the adjustment bolt 300 is thereafter caused to be rotated until the clutch face 310 at the left end of the adjustment bolt 300 comes into the clutch engagement and screwed out from the piston 200. Accordingly, the retracted position of the piston 200 may be set in response to the amount of the lining wear.
As is evident from the above-described operation of the automatic shoe clearance adjustment, the locator spring 370 positioned between the facing large diameter bores 360, 360 of the pair of adjustment bolts 300, 300 is constantly urging the adjustment bolts 300, 300 in the axial direction in order to prevent the free movement due to vibration caused while in braking operation. However, because the torsion force of the locator spring 370 acting on the adjustment bolt 300 may result in unstable automatic shoe clearance adjustment operation, a conventional device provides the locator 380 and the spacer 390 between one end of the locator spring 370 and the stepped surface 351. Here, the conventional device has the following points to be improved.
In order to act the force of the locator spring 370 to the axis center of the adjustment bolt 300, the spacer 390 with a concave portion guiding the top of the locator 380 into the axis of the adjustment bolt 300 is used. However, this concave portion is provided only at one surface of the spacer 390, there is a possibility of misassembling the spacer 390 into the large diameter bore 360 of the adjustment bolt 300.
The piston cup 400, the dust boot 420, and the O-ring 430 used in the cylinder device are rubber or elastomeric members and need to be replaced periodically. Accordingly, there is a possibility of loosing the members or omitting a particular member during assembly and reassembly in addition to the above-described misassembly of the spacer 390 during reassembly.
This invention is made to improve the above-described points and is to provide a brake cylinder device with a built-in automatic shoe clearance adjustment mechanism enable to eliminate the possibility of loosing the members, omitting the particular member, and misassembling.
The present invention is directed to a brake cylinder device with a built-in automatic shoe clearance adjustment mechanism. The device includes a pair of opposing pistons slidably fit in a cylinder bore of a cylinder body and a pair of adjustment bolts having a coaxial through hole. One end of each adjustment bolt is screwed into the piston with a non-reversible screw thread connection. The other end of each adjustment bolt is urged into clutching engagement with the cylinder body. A pair of drive rings, internally threadingly engage the other side of the adjustment bolt in a reversible screw thread connection with a backlash in the axial direction. The peripheral surface of the drive ring is urged by an adjustment spring into clutching engagement with a partition of the cylinder body. The pistons, adjustment bolts, adjustment springs, and drive rings are provided oppositely and symmetrically in the cylinder bore. A locator spring is positioned between two facing adjustment bolts. A locator is placed in the vicinity of the end of the locator spring. A spacer is placed between the top end of the locator and one of the adjustment bolts, having a supporting portion which engages with and supports the top end of the locator along the axis of the adjustment bolt. A fluid flow passage penetrates through the spacer, wherein the supporting portions formed on the axial center of the spacer are identically shaped both on the front and back surfaces thereof.
The top end of the locator has a conical shaped portion (convex shape) and the supporting portion of the spacer has a tapered concave portion. The spacer supports the top end of the locator and the conical shaped portion is disposed within and engages the tapered concave portion of the space. An angle of the tapered concave portion is designed to be larger than an angle of the conical shape top end of the locator to define a point contact between the conical shaped top end and the tapered concave portion.
The locators are preferably positioned one each adjacent side of the locator spring and the spacers are disposed between the top end of both locators and an associated adjustment bolt. The spacer is preferably symmetrically formed to have front and back surfaces of the same shape. The spacer is integrally pressed in the through hole of the adjustment bolt and is substantially integrated therewith. The outer circumferential diameter on the back and front surfaces of the spacer is preferably designed to be smaller than an inner diameter of the through hole of adjustment bolt. The locator is urged by the locator spring such that the top end of the locator and the supporting portion of the spacer are aligned with the axis of the adjustment bolt.